The present invention relates to a device for converting light from one wavelength to another.
It is known that when two beams of light are directed into a nonlinear optical crystal, a third beam can be generated, the frequency of the third beam being equal to the sum or difference of the frequencies of the first two beams. This process is referred to as sum frequency generation (SFG) or difference frequency generation (DFG). Difference frequency generation in particular is useful in wavelength-division multiplexing (WDM), for converting the wavelength of a beam of signal light to another nearby wavelength.
One requirement for efficient difference frequency generation is that the two incident beams maintain a high intensity during their passage through the crystal. This requirement can be met by confining the beams in a waveguide in the crystal, but coupling two incident beams of different wavelengths into the same waveguide efficiently is difficult, as the beams are refracted differently by the coupling lens.
An alternative scheme that has become known in recent years employs Bessel beams, also referred to as non-diffracting beams or diffraction-free beams. Bessel beams can be produced by passing the incident light through an axicon lens.
Another requirement for difference frequency generation is the phase matching condition. If this condition is not satisfied, converted light generated at different points in the nonlinear optical crystal will interfere destructively, so that little or no converted light is obtained in the end. The phase-matching condition is difficult to satisfy, because the refractive index of the nonlinear optical crystal varies with wavelength, and differs for each of the three beams involved in the DFG process.
One conventional method of satisfying the phase matching condition employs an anisotropic nonlinear optical crystal, the refractive index of which also varies depending on the direction of propagation and direction of polarization of the beams. If one of the three beams is polarized in one direction and the other two beams are polarized in a perpendicular direction, the anisotropy can correct for the effect of the wavelength dependency of the refractive index. This method is referred to as type I or type II phase matching, according to the particular combination of polarizations employed.
Another conventional method of phase matching, referred to as quasi phase matching (QPM), provides a periodic domain inversion structure in the nonlinear optical crystal. This structure allows a phase mismatch to occur, but periodically corrects the mismatch, so that the mismatch never becomes very large.
Maximum conversion efficiency cannot be achieved with any of these conventional methods, however.
With type I and type II phase matching, the reason is that the strength of the nonlinear effect that produces wavelength conversion varies according to the direction of polarization. Since the three beams are polarized in two different directions, it is not possible to employ the single most efficient polarization direction.
With quasi phase matching, conversion efficiency is reduced by (2/.pi.).sup.2.